1. Technical Field
The present invention relates in particular to a travel control apparatus for a vehicle, which enables the vehicle to travel with stability even when a rut exists in a road.
2. Related Art
Recently, various apparatuses employing automatic driving techniques that enable a driver to drive a vehicle in increased comfort and more safely have been developed and proposed. Japanese Unexamined Patent Application Publication (JP-A) No. 2014-184747, for example, discloses a technique employed in a travel control apparatus for a vehicle that detects a travelable region of a vehicle and executes locus control on the basis of a target vehicle behavior amount calculated to enable the vehicle to travel through the travelable region. According to this technique, when the travelable region is a curve having a curvature and a rut is detected on an inner side of the curve, the target vehicle behavior amount is corrected so that an inner wheel of the vehicle travels in the detected rut on the inner side of the curve.
The technique for a vehicle control apparatus disclosed in JP-A No. 2014-184747 is effective in improving the vehicle behavior and the stability of steering control while travelling along a rut. However, the rut condition of a road varies constantly, and the vehicle behaves highly erratically whenever the vehicle falls into and pulls out of a rut. It is therefore preferable, in terms of maintaining continuous stability in the vehicle behavior, to ensure that the vehicle does not repeatedly fall into and pull out of a rut while travelling. In addition to the problems arising when the vehicle falls into and pulls out of a rut, during travel within a particularly narrow rut having a width no greater than that of a single tire, a ground contact condition of the tire may become unstable, with the result that desired braking and driving force cannot be generated. Furthermore, even when the technique for a vehicle control apparatus disclosed in JP-A No. 2014-184747 is employed during travel in a normal rut, a road surface friction coefficient difference may occur between the rutted road surface and the non-rutted road surface, and as a result, a yaw moment may be generated during braking or acceleration, leading to instability in the vehicle behavior.